Synthesis gases containing carbon monoxide and hydrogen as principal ingredients are being widely utilized as source materials for producing dimethyl ether, methanol, Fischer-Tropsch oil, acetic acid, diphenymethane diisocyanate, methyl methacrylate and so on. Techniques for producing such synthesis gases include, for example, carbon dioxide reforming of causing methane-containing light hydrocarbons and carbon dioxide to react with each other in the presence of a catalyst, steam reforming of causing light hydrocarbons containing methane etc. and steam to react with each other in the presence of a catalyst and carbon dioxide/steam reforming of causing light hydrocarbons containing methane etc., carbon dioxide and steam to react with one another in the presence of a catalyst.
Carbon dioxide reforming and carbon dioxide/steam reforming are accompanied by a problem of causing a side reaction of carbon deposition to take place due to the source materials of light hydrocarbons and the reforming product of carbon monoxide. A phenomenon of catalyst poisoning occurs as carbon deposits on the catalyst to damage the catalytic activity and reduce the reaction rate. With either of these techniques, therefore, it is difficult to efficiently produce synthesis gas on a stable basis for a long duration of time. Additionally, carbon deposition can cause a rise of pressure difference and blockage to occur in the reforming reactor.
To cope with the problem of carbon deposition, there have been disclosed a carbon dioxide reforming catalyst prepared by causing a carrier made of at least one or more of alkaline earth metal oxides and aluminum oxide to carry a ruthenium compound (see PTL 1 listed below), a carbon dioxide reforming catalyst prepared by causing a carrier made of an oxide of Groups II through IV metals or an oxide of lanthanoid metals or a carrier made of an alumina complex containing an oxide of such metals to carry rhodium (see PTL 2 listed below) and a synthesis gas production catalyst prepared by causing a carrier made of a metal oxide to carry at least a catalyst metal selected from rhodium, ruthenium, iridium, palladium and platinum and showing a specific surface area of not greater than 25 m2/g, an electronegativity of the metal ions in the carrier metal oxide of not greater than 13.0 and a catalyst metal carrying rate between 0.0005 and 0.1 mol % relative to the carrier metal oxide in terms of metal (see PTL 3).
However, there is still a demand for catalysts that can further suppress carbon deposition and operate to efficiently produce synthesis gas on a stable basis for a long duration of time.
Additionally, with carbon dioxide/steam reforming, the ratio of the source gas and the produced synthesis gas changes as a function of the abundance ratio of carbon dioxide to hydrocarbons, or CO2/C, and that of steam to hydrocarbons, or H2O/C, in the source gas. FIG. 1 shows the relationship between the abundance ratio (molar ratio) of carbon dioxide (CO2) and steam (H2O) to carbon (C) and the ratio (volume ratio) of the source gas to the produced synthesis gas. From the viewpoint of synthesis efficiency, it is desirable to conduct a reforming operation with a CO2/C ratio and a H2O/C ratio that minimize the ratio of the source gas to the produced synthesis gas. This is because such an arrangement can provide advantages of downsizing the reformer apparatus and reducing the running cost (the quantity of the source gas) among others.
However, with a CO2/C ratio and an H2O/C ratio that minimize the ratio of the source gas to the produced synthesis gas, carbon can deposit on the catalyst surface to a large extent to make it difficult to effectively conduct carbon dioxide/steam reforming operations.